1. Field of the Invention
The present invention relates generally to detecting slag in a stream of molten steel or the like and, more particularly, is concerned with the design of the slag detector transducer coil assembly.
2. Description of the Prior Art
In the continuous casting of steel billets, blooms, and slabs, steel is transferred from a ladle to a tundish in a continuous stream via gravity feed through a refractory shroud. A valve in the bottom of the ladle is used to terminate the steel flow as the ladle empties and another ladle is brought into position to keep the tundish replenished with molten steel. Each ladle of molten steel is referred to as a "heat", and many heats are required for one continuous casting run. In order to maintain high quality and a uniform cast product, it is important to maintain a uniform quality of molten steel in the succession of heats.
Slag, which consists of various oxides created in the furnace and ladle, has a lower density than steel and consequently floats on the steel surface. Since steel is withdrawn from the ladle's bottom, slag is kept from contaminating the tundish and the finished product. This technique works until near the end of each heat, when slag tends to mix with steel due to vortexing effects created by the steel discharge. In order to minimize contamination, the level of steel in the ladle is monitored visually and the flow is terminated when it appears to be near the onset of vortexing; i.e., slag entrainment, in the discharge stream.
Usually, the flow is terminated early, and valuable steel is subsequently scrapped along with slag for recycling in the furnace. Occasionally flow is not terminated in time, and large volumes of slag are sucked into the discharge and into the tundish. Since there is no reliable means available for measuring slag content in the discharge stream, the process is strictly a matter of judgement based upon experience, and the tendency is to maintain quality of the finished product by cutting flow of uncontaminated steel. For a typical slab caster, the net worth of disposed steel from each heat is worth several hundred-thousand to several million dollars annually.
Novel means for detecting the onset of slag entrainment by using a transducer coil which is mounted coaxially with the shroud transferring the molten steel discharge from the ladle to the tundish are disclosed in U.S. Patent Nos. 4,523,146; 4,590,424 and 4,635,832. But prior art structures and methods for detecting slag in molten metal with a coil during pouring of the metal have not dealt with a dependable and durable coil construction.
A transducer coil which is mounted co-axially with the molten steel discharge stream flowing through a refractory shroud is used to detect the onset of slag entrainment in the steel stream. Since the temperature of the outer surface of the shroud can reach at least 1300.degree. F., and it is desirous to construct the coil as compactly as possible to keep its resistance low, it becomes imperative to design the coil assembly to minimize its temperature environment. The necessity for minimizing the coil temperature is two-fold, firstly, to keep its resistance low since the principal of the slag detection scheme depends upon the measurement of the percentage change of the coil resistance, and secondly, so that the coil winding insulation can survive during the lifetime of the slag detector assembly.